(1) Field of the Invention
The invention relates to improved electrodes for use in electrolytic cells utilizing alkaline electrolytes.
(2) Description of the Prior Art
In an electrochemical cell having as basic components at least one anode and one cathode and an electrolyte, a chemical reaction may be achieved such as the oxidation or reduction of a chemical compound, as in an electrolytic cell or the conversion of chemical energy in a fuel into a low voltage direct current, as in a fuel cell. When the electrodes in such a cell are of relatively inexpensive material such as for instance iron or nickel, the electrodes tend to have low activity. The problem is particularly acute in electrochemical cells used, for example, in the electrolysis of water to produce hydrogen and oxygen utilizing an alkaline electrolyte (for instance a 25% aqueous solution of potassium hydroxide).
The use of nickel as an anode material for commercial water electrolyzers is unsatisfactory because the over-voltage for oxygen evolution on nickel is high and increases with length of service. Electrode coatings of mixed ruthenium-titanium oxides are useful for the production of oxygen in acidic solutions but the chemical stability of such anodes in a strongly alkaline environment, as used in water electrolyzers, is inadequate. Graphite which is useful as an anode for chlorine production is rapidly destroyed by oxygen if used for water electrolysis.
U.S. Pat. No. 4,342,792, electrocatalysts are disclosed which can be coated over a metal electrode substrate to provide an electrode of high activity and stability when used as an anode in a strongly alkaline electrolyte. Such anodes are produced by coating said electrode substrate with a homogeneous solution of a mixture of (1) at least one compound from the group of compounds consisting of iron, cobalt, nickel, and manganese, (2) at least one compound from the group of compounds consisting of molybdenum, tungsten, and vanadium, and (3) at least one rare earth metal selected from the compounds of lanthanides having an atomic number of 57-71 inclusive. When such compounds are coated on an electrode substrate, if such compounds are not oxides, the compound must be capable of thermodecomposition to the corresponding metal oxide. The oxide coated substrate is thereafter cured in a reducing atmosphere.
In U.S. Pat. No. 4,428,805, electrodes for oxygen manufacture are disclosed. The electrodes are prepared by coating an electroconductive substrate with a first coating of one or more oxides of the metals tin, lead, antimony, aluminum, and indium followed by a second coating of a monometal or a polymetal oxide having a spinel structure.
In U.S. Pat. No. 4,464,239, lithiated cobalto-cobaltic oxides are used as coatings for electrode substrates as a means for reducing the electrode over-voltage in a water electrolysis cell having an alkaline electrolyte.
In European Patent Publication No. 0,009,406, electrodes are disclosed having electrocatalytic coatings of the nickel-molybdenum type including mixtures of cobalt and tungsten. Such electrodes are coated on electrode substrates such as nickel, iron, copper, and titanium and their alloys from a solution of compounds of these metals. The compounds used must be capable of thermal decomposition to their oxides. Subsequently, the oxide coated substrate is cured in a reducing atmosphere.